1. Field of the Invention
The present invention relates to a method of treating an oil-containing waste water, and particularly to such a method of treating an oil-containing waste water in which one or more fouling components including oil are finely dispersed in water by a surfactant, and thereby separating the fouling components and the water from each other.
2. Discussion of Related Art
Generally, in machine factories in which respective sorts of machines are used, or in cleaning shops, articles bearing fouling components including oil are washed or cleaned using water, a surfactant, and an alkaline agent, and a large amount of oil-containing waste water in which the fouling components are dispersed in the water by the surfactant is produced. The oil-containing waste water is subjected to a treatment in which the fouling components including oil and the water are separated from each other. Then, the fouling components are removed from the waste water, and only the water is discharged into the environment.
Since, in the above-indicated oil-containing waste water, the fouling components are dispersed in the form of very fine particles in the water by the surfactant, such a treating method has been conventionally used in which, after a preliminary treatment in which a screen filter or the like is used to remove solid matters suspended in the waste water, three steps are carried out to separate the fouling components and the water from each other.
More specifically explained, after the above-described preliminary treatment, the first step is carried out in which an appropriate amount of dilute sulfuric acid is added to the oil-containing waste water, so that a pH value of the waste water is made not greater than three and a major portion of the oil is separated from the fouling components owing to a decomposing effect of the dilute sulfuric acid. The thus separated major portion of the oil is removed from the waste water. Subsequently, the second step is. carried out in which a neutralizer such as sodium hydroxide is added to the waste water from which the major portion of oil has been removed, so as to neutralize the waste water. In addition, aluminum sulphate, PAC, or emulsion breaker is added to the neutralized waste water so as to form fine flocks consisting of the remaining portion of oil dispersed in the waste water and the other fouling components than the oil. Moreover, an appropriate amount of high-molecular flocculant or coagulant is added to the waste water in which the fine flocks have been formed, such that a concentration of the flocculant in the waste water ranges from 10 to 50 mg/L(liter). Thus, the formation of the fine flocks is promoted and the fine flocks are grown into a sludge. The waste water in which the sludge has been formed is subjected to a known floatation separation method in which the sludge is caused to float on the surface of the waste water. This sludge is removed from the waste water. Then, the third step is carried out in which an inorganic adsorbent such as bentonite is added to the waste water and additionally an appropriate amount of high-molecular flocculant is added to the waste water so that a concentration of the flocculant in the waste water is about 20 mg/L(liter). Thus, the remaining portion of oil and the other fouling components dispersed in the waste water are flocculated or coagulated in a state in which the oil and the other fouling components are adsorbed by the inorganic adsorbent, and accordingly an additional amount of sludge is formed. This sludge is removed by the same method as used in the second step. In this way, the oil-containing waste water is completely separated into the fouling components including oil, and the water. This oil-containing waste water treating method is disclosed in, e.g., JP-A-5-285305.
However, in the above-explained, conventional oil-containing waste water treating method, the oil-containing waste water is treated using the respective sorts of agents and/or chemical compounds in the three steps. Thus, the conventional method is not economical, and is complicated as a whole. In addition, since the three steps use respective different treatment equipment, the overall cost needed to prepare such equipment is significantly increased, and the footprint needed to dispose all of them is large-scaled. Moreover, in the conventional method, the respective steps produce the oil or the sludge that contains the treating agent and/or chemical compound added to the waste water. Thus, the total amount of oil and sludge produced by the three steps is inevitably increased, and additional cost and labor are needed to treat the increased amount of oil and sludge.